1. Technical Field of the Invention
This invention relates to compositions useful in the preparation of semiconductive conductor shields in power cables and to semiconductive conductor shields and power cables utilizing the composition.
2. Description of the Related Art
A typical insulated electric power cable generally comprises a conductor in a cable core that is surrounded by several layers of polymeric materials including an inner semiconducting shield layer (conductor or strand shield), an insulating layer, an outer semiconducting shield layer (insulation shield), a metallic wire or tape shield used as the ground phase, and a protective jacket. Additional layers within this construction such as moisture impervious materials, are often incorporated. The invention pertains to the inner semiconducting shield layer, i.e., the conductor shield.
Semiconductive shields have been used in power cables as shields for the cable conductor and insulation for many years. The conductor shield is typically extruded over the cable conductor to provide a layer of intermediate conductivity between the conductor and cable insulation in the power cable. Conventional compositions for these conductor shields include a base polymer as the predominant component of the composition compounded with, carbon black to provide conductivity for the composition and various additives.
The primary purpose of the semiconducting conductor shield between the conductor and insulation in an electrical power cable is to ensure the long term viability of the primary insulation. There is always a need for improved semiconductive conductor shield compositions that balance cost and performance.
Examples of polymer compositions used as shields in power cables are found in the disclosures of U.S. Pat. Nos. 4,612,139 and 4,305,846 to Kawasaki et al., U.S. Pat. No. 4,857,232 to Burns, Jr., U.S. Pat. No. 3,849,333 to Lloyd et al., U.S. Pat. No. 5,889,117 to Flenniken, and U.S. Pat. No. 6,086,792 to Reid et al., the disclosures of which are hereby incorporated by reference.
It would be desirable to have a conductor shield material with improved performance that does not require the use of expensive additives, complex polymer formulations, or specially prepared carbon black, as performance must always be balanced with cost in the manufacture of electric cable.
The invention provides a conductor shield material with improved performance without the need for expensive additives, complex polymer formulations, or specially prepared carbon black.
In particular, the composition of the invention, conductor shields and cables made with conductor shields in accordance with the invention exhibit superior performance over time as demonstrated by accelerated cable life testing (ACLT) as compared to conventional high performance conductor shield compositions.
In particular, the invention provides a conductor shield comprising a base polymer selected from the group consisting of copolymers of ethylene and a mono-unsaturated ester, copolymers of ethylene and one or more alpha olefins having three to six carbon atoms, EPR and EDPM rubbers, low density polyethylene and linear low density polyethylene; conductive carbon black; and a waxy additive selected from the group consisting of at least one amide wax, at least one ethylene vinyl acetate wax and mixtures of at least one amide wax and at least one ethylene vinyl acetate wax.
The invention also provides a conductor shield consisting essentially of a base polymer selected from the group consisting of copolymers of ethylene and a mono-unsaturated ester, copolymers of ethylene and one or more alpha olefins having three to six carbon atoms, EPR and EDPM rubbers, low density polyethylene and linear low density polyethylene; conductive carbon black; and a waxy additive selected from the group consisting of at least one amide wax, at least one ethylene vinyl acetate wax and mixtures of at least one amide wax and at least one ethylene vinyl acetate wax.
In addition to the composition matter, the invention includes a semiconductive shield for the conductor or insulation in a power cable formed by extruding the composition over the conductor or insulation of the power cable and the resulting power cable that employs the composition as a conductor shield.
The base polymer of the composition of the invention can be selected from a variety of polymers including various homopolymers, copolymers and terpolymers known in the art, the selection being based upon the ultimate desired use of the polymer composition. For example, the polymers used in the polymeric compositions of the present invention may include, but are not limited to, homopolymers, copolymers and graft polymers of ethylene where the co-monomers are selected from butene, hexene, vinyl acetate, acrylic acid, methacrylic acid, esters of acrylic acid, esters of methacrylic acid, maleic anhydride, half esters of maleic anhydride, carbon monoxide and the like; elastomers selected from natural rubber, polybutadiene, polyisoprene, random styrene butadiene rubber, polychloroprene, nitrile rubbers, ethylene propylene copolymers and terpolymers and the like; homopolymers and copolymers of styrene, including styrene-butadiene, styrene-butadiene-styrene linear and radial polymers, acrylonitrile-butadiene-styrene, styrene acrylonitrile and the like; linear and branched polyether or polyester polyols; crystalline and amorphous polyesters and polyamides; alkyd resins, rosin acids or rosin esters; hydrocarbon resins produced from thermal or Friedal Crafts polymerization of cyclic diene monomers such as dicyclopentadiene, indene, cumene and the like; ethylene/silane copolymers; ethylene/.alpha.-olefin/diene terpolymers such as ethylene/propylene/1,4-hexadiene, ethylene/1-butene/1,4-hexadiene and the like; mixtures thereof and the like. Additionally, the polymer used in compositions of the present invention may include copolymers and terpolymers containing the above-identified polymers as major components of the copolymer or terpolymer.
Preferably, the base polymer of the composition of the invention is selected from a variety of polymers including copolymers of ethylene and a mono-unsaturated ester such as ethylene-ethyl acrylate, ethylene-methyl acrylate, ethylene-methyl methacrylate and ethylene-vinyl acetate, copolymers of ethylene and one or more alpha olefins having three to six carbon atoms, as well as EPR and EDPM rubbers, low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). Of these copolymers, ethylene-vinyl acetate (EVA) is more preferred. More particularly, EVA having a vinyl acetate content between 18 and 20% is most preferred for use as the base polymer of the invention. The base polymer of the composition of the invention is present in amounts from about 30% to about 99.4% by weight, based upon the weight of the of the total composition.
In the present invention, commercially available, conventional carbon black is added to the polymer compositions to impart semi-conductive properties to the composition. The ability to use such commercially available, conventional carbon blacks to achieve improved ACLT results is an advantage of the invention. The carbon black added to the polymer may be one of the various available conventional carbon blacks, including finely divided carbon such as lamp black, furnace black, or acetylene black, i.e. carbon black made by pyrolyzing acetylene. Ketjin black may be used in the compositions of the invention as well as many of the commercial carbon black grades described in ASTM D 1765 98b, for example, N351, N293 and N550. Preferably, to avoid problems associated with carbon black dust, the carbon black is pelletized, although non-pelletized carbon black, such as in its fluffy form, may also be used with equal success. The carbon black is generally present in the composition in the amount of from about 0.1% to about 65% by weight of the polymer composition. Preferably the carbon black is present in an amount of from about 10% to about 50% by weight, based on the weight of the total composition.
A tremendous number of compounds have been suggested for use as additives in semiconducting shield compositions. Typically, these compounds fall into the category of antioxidants, curing agents, vulcanizing agents, crosslinking agents, boosters and retardants, processing aids, pigments, dyes, colorants, fillers, coupling agents, ultraviolet absorbers or stabilizers, antistatic agents, nucleating agents, slip agents, plasticizers, lubricants, viscosity control agents, tackifiers, anti-blocking agents, surfactants, extender oils, acid scavengers, and metal deactivators.
The present invention is based upon the discovery that certain polar waxes produce a shield composition having enhanced electrical aging performance as measured by accelerated cable life testing (ACLT). The polar waxes of the invention, i.e., the waxy additive, are selected from the group consisting of at least one amide wax, at least one ethylene vinyl acetate wax and mixtures of at least one amide wax and at least one ethylene vinyl acetate wax. In embodiments of the invention the amide wax is selected from stearamide, oleamide, erucamide, ethylene bis-stearamide, ethylene bis-oleamide, ethylene bis-erucamide and mixtures thereof. Oleamide and erucamide are preferred. Oleamide is commercially available from Croda Universal, Inc. under the designation CRODAMIDE OR(copyright) and euracamide is commercially available from Croda Universal, Inc. under the designation CRODAMIDE ER(copyright). In other embodiments of the invention, the waxy additive is an ethylene vinyl acetate (EVA) wax and has a molecular weight from about 15,000 Daltons to about 40,000 Daltons and a vinyl acetate content of from about 2% to about 28%, preferably from about 10% to about 20%. In more preferred embodiments, the EVA wax has a molecular weight from about 15,000 Daltons to about 30,000 Daltons and a vinyl acetate content of from about 12% to about 15%. The waxy additive may be at least one amide wax, at least one EVA wax, or a mixture of at least one amide wax and at least one EVA wax. The waxy additive is from about 0.5% to about 5.0% by weight of the composition, preferably from about 0.8% to about 2.0% by weight of the composition.
Examples of antioxidants are as follows, but are not limited to: hindered phenols such as tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydro-cinnamate)] methane; bis[(beta-(3,5-ditert-butyl-4-hydroxybenzyl)-methylcarboxyethyl)]sulphide, 4,4xe2x80x2-thiobis(2-methyl-6-tert-butylphenol), 4,4xe2x80x2-thiobis(2-tert-butyl-5-methylphenol), 2,2xe2x80x2-thiobis(4-methyl-6-tert-butylphenol), and thiodiethylene bis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate; phosphites and phosphonites such as tris(2,4-di-tert-butylphenyl)phosphite and di-tert-butylphenyl-phosphonite; thio compounds such as dilaurylthiodipropionate, dimyristylthiodipropionate, and distearylthiodipropionate; various siloxanes; polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), n,nxe2x80x2-bis(1,4-dimethylpentyl-p-phenylenediamine), alkylated diphenylamines, 4,4xe2x80x2-bis(alpha, alpha-demthylbenzyl)diphenylamine, diphenyl-p-phenylenediamine, mixed di-aryl-p-phenylenediamines, and other hindered amine antidegradants or stabilizers. Antioxidants can be used in amounts of about 0.1 to about 5 percent by weight based on the weight of the composition.
Examples of curing/crosslinking agents are as follows: dicumyl peroxide; bis(alpha-t-butyl peroxyisopropyl)benzene; isopropylcumyl t-butyl peroxide; t-butylcumylperoxide; di-t-butyl peroxide; 2,5-bis(t-butylperoxy)2,5-dimethylhexane; 2,5-bis(t-butylperoxy)2,5-dimethylhexyne-3; 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane; isopropylcumyl cumylperoxide; di(isopropylcumyl) peroxide; or mixtures thereof. Peroxide curing agents can be used in amounts of about 0.1 to 5 percent by weight based on the weight of the composition.
The polymer compositions of the present invention may be manufactured using conventional machinery and methods to produce the final polymer product. The compositions may be prepared by batch or continuous mixing processes such as those well known in the art. For example, equipment such as Banbury mixers, Buss cokneaders, and twin screw extruders may be used to mix the ingredients of the formulation. The components of the polymer compositions of the present invention may be mixed and formed into pellets for future use in manufacturing electrical cable.
The composition of the invention, and conductor shields and cables made with conductor shields in accordance with the invention exhibit superior performance over time as demonstrated by accelerated cable life testing (ACLT) as compared to conventional high performance conductor shield compositions. Specifically, as described below in the Example, the composition of the invention when used in a conductor shield has an accelerated cable life testing Weibull Beta value of 1.5 or more, preferably 3.0 or more. While the connection between smoothness of the conductor shield may or may not be related to the improved ACLT values, nonetheless, the composition of the invention when used in a conductor shield may achieve a count of surface imperfections/m2 of 500 or less, preferably a count of surface imperfections/m2 of 200 or less.